1. Field of the Invention
The present invention relates to an image pickup apparatus.
2. Description of the Related Art
In recent years, the popularity of an interchangeable-lens camera such as a single-lens reflex camera in which, an electronic image pickup apparatus such as a CCD (charge coupled device) and a CMOS (complementary metal oxide semiconductor) is used, has been increasing. In an interchangeable lens which is used in the interchangeable-lens camera, making a focusing lens group light-weight and simple and enabling rear focus by the focusing lens group have been sought. This is because, by making the focusing lens group light-weight and enabling rear focus for reducing drawing out of the interchangeable lens, it is possible to reduce a sound at the time of focusing. Accordingly, it is possible to reduce a sound of focusing at the time of still photography, and a sound of focusing which is generated by a frequent focusing movement at the time of video photography, in a case such as of a video photography by the interchangeable-lens camera of recent years in which, a highly defined video-photography function surpassing the conventional digital video camera has been installed.
However, when the focusing lens group is merely made light-weight and simple, it is not possible to prevent deterioration of various aberrations caused due to focusing. For instance, when focusing is carried out by one lens, suppressing fluctuation of aberrations, mainly a chromatic aberration, becomes difficult. Moreover, improving Petzval's sum also becomes difficult.
As a telescopic image forming optical system, image forming optical systems of a so-called double telephoto type, which include four lens groups namely, a positive lens group, a negative lens group, a positive lens group, and a negative lens group as disclosed in Japanese Patent Application Laid-open Publication Nos. Sho 60-142319, Sho 60-55314, Hei 1-223409, and Hei 4-53916, have been proposed. Moreover, in Japanese Patent Application Laid-open Publication No. Hei 4-53916, using a third lens group having a three-lens structure as a focusing lens group has been proposed.